TECHNICAL FIELD
[0001] The present invention relates to a method for regenerating a filter, and it is particularly
related to the method for regenerating a filter which has been used in a filtration
step in the process for producing an acrylic acid.
BACKGROUND ART
[0002] The process for producing an acrylic acid via a catalytic gas phase oxidation process,
an absorption and/or a condensation process,andfurther a distillation purification
and/or a crystallization process has been industrially performed widely. Purified
acrylic acid is transferred and stored in a storage tank, and if a trace of acrylic
acid polymer is contained in the purified acrylic acid, an acrylic acid polymer will
be further generated during storage, and problems such as the decreasing of purity
of the purified acrylic acid are caused.
[0003] In order to solve the above problems, the present inventors have previously proposed
a method for filtering with a filter when the purified acrylic acid is transferred
to a storage tank. (Refer to patent document 1). In saidmethod, in order to remove
the acrylic acid polymer adhered to the used filter, the filter was washed with aqueous
sodium hydroxide solution after water washing, and further water washing was carried
out (paragraph [0041]).
[0004] The acrylic acid polymer adhered on the filter can be substantially removed by washing
with water and aqueous sodium hydroxide solution, and performing further water washing
as described in the above patent document 1. It should be noted that a final water
washing is performed for removing the alkaline residues which remain on the filter
after washing with aqueous sodium hydroxide solution.
PRIOR ART REFERENCES
[Patent literatures]
SUMMARY OF INVENTION
TECHNICAL PROBLEM
[0006] However, present inventors found that when the filter regenerated by the above methods
was re-used for filtration of the purified acrylic acid, the color tone of the purified
acrylic acid after restarting the filtration was bad, and the metal (mainly iron)
content was high, and the metal had affected the color tone of the purified acrylic
acid. Moreover, it was found that, by passing some amount of the purified acrylic
acid through a filter, the color tone and metal content of the purified acrylic acid
could return to APHA 5 or less and the metal content of 1 ppm by mass or less, which
is the value in the purified acrylic acid immediately after a crystallization process
and is the usual value generally required for the product acrylic acid. However, till
the color tone and metal contents of the purified acrylic acid return to the usual
value, it is necessary to passing a large amount of the acrylic acid for regeneration
through the filter. As the result, the quantity of the off-specification product of
acrylic acid increases. Although the acrylic acid, which is said off-specification
product, may be recycled for a recovery step, another crystallization step, etc.,
the production cost of acrylic acid become expensive.
[0007] In recent years, because a high qualityproduct of acrylic acid with a good color
tone has been required, solution of the above problems is strongly desired. In addition,
it is also desirable from the point of cost reduction to make the amount of off-specification
product as less as possible.
[0008] From this viewpoint, it is an object of the present invention to provide a method
for regenerating the filter which prevents the color tone of the purified acrylic
acid from deteriorating.
MEANS FOR SOLVING THE PROBLEM
[0009] The present inventors have investigated in detail and found that the above metal
contained a metal (mainly iron) that was slightly eluted from the reactor used in
the process for producing acrylic acid. That is, the metal remains on the filter as
an oxide when the filter is washed with aqueous alkaline solution, and when said filter
is re-used, the metal is eluted into the purified acrylic acid and causes a deterioration
of the color tone of the purified acrylic acid. In order to resolve the above causes,
the present inventors have intensively studied and completed this invention.
[0010] That is, the present invention is a method for regenerating a filter which has been
used in a filtration step in the process for producing acrylic acid, comprising a
step (A) wherein the filter is washed with an aqueous alkaline solution, a step (B)
wherein the filter is washed with water after the step (A), and a step (C) wherein
the filter is brought into contact with acrylic acid for regeneration for at least
one hour after the step (B).
ADVANTAGEOUS EFFECT OF THE INVENTION
[0011] The present invention can prevent deterioration of the color tone of the purified
acrylic acid.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0012] Hereinafter, the preferred embodiments of the present invention is explained.
[0013] The present invention is a method for regenerating a filter which has been used in
a filtration step in the process for producing acrylic acid, comprising a step (A)
wherein the filter is washed with an aqueous alkaline solution, a step (B) wherein
the filter is washed with water after the step (A), and a step (C) wherein the filter
is brought into contact with acrylic acid for regeneration for at least one hour after
the step (B).
[0014] The process for producing acrylic acid of the present invention is not particularly
limited. For example, there is included a process which comprises of a process wherein
acrylic acid is produced via a catalytic gas phase oxidation process, a absorption
and/or a condensation process, and further distillation purification and/or crystallization
process. It should be noted that a "crystallization process" is a well-known process
for purifying acrylic acid by crystallizing, sweating and melting.
[0015] The filtering process of the present invention is performed in order to remove the
acrylic acid polymer included in the purified acrylic acid, and it is performed with
a filter. The filtering process will not be particularly limited as long as it is
carried out in the process for producing the acrylic acid, but there is included the
case where it is carried out when transferring the purified acrylic acid obtained
from a distillation purification and/or crystallization process to storage tanks thereof,
or the case where it is carried out when transferring to another storage tank installed
as needed, before supplying the acrylic acid solution obtained from the absorption
and/or condensation process to a distillation purification and/or crystallization
process. Especially, it is preferable to be performed in the line during the transfer
of the purified acrylic acid obtained from a crystallization process to a storage
tank thereof.
[0016] The kind, shape or material etc. of the filter to be regenerated in the present invention
is not especially limited, any one will be enough as long as it is able to be used
to remove the acrylic acid polymer. Especially, a commercially available cartridge
type filter is suitably employed for convenience of operation. Specifically, there
are included a membrane cartridge filter made of polytetrafluoroethylene (PTFE), a
pleats cartridge filter made of polypropylene (PP), a depth cartridge filter, a wound
cartridge filter, and a pleats cartridge filter made of stainless steel etc. The present
invention is especially effective in regeneration of a cartridge filter made of resin
such as PTFE and PP, which cannot be reversely washed in terms of its strength.
[0017] As for the method for regenerating the filter, a filter housing that storing a cartridge-type
filter is used alone, or two or more filter housing that storing a cartridge-type
filter can be used in the any form disposed in tandem and/or in parallel. Preferably,
two filter housing (I) and (II) are installed in parallel, and at first, the purified
acrylic acid is flowed into filter housing (I) and then filtered. Then, if the differential
pressure onto a filter increases due to adhesion of the polymer onto the filter, the
flow of the purified acrylic acid will be changed to filter housing (II), and filtration
of the purified acrylic acid will be continued. The filter in the above filter housing
(I) is regenerated, for example, as follows, though it is not especially limited to
the following order.
- 1) The acrylic acid which remained in the housing is removed and recovered.
- 2) The inside of the housing is washed with water, and the residual acrylic acid is
flushed out [a step (X)].
- 3) An aqueous alkaline solution is filled in the housing and the polymer adhered to
the filter is removed [a step (A)].
- 4) After the aqueous alkaline solution is removed from the housing, the inside of
the housing is washed with water, and the alkaline is flushed out [a step (B)].
- 5) The inside of the housing is filled with the acrylic acid for regeneration, and
a filter is brought in contact with the said acrylic acid [a step (C)].
[0018] The above step 1) may be a general removal and/or a recovery, for example, flushing
out is included. The recovered acrylic acid can be circulated to the absorbing step
of acrylic acid.
[0019] In the above step 2), the pH of water after flushing out the residual acrylic acid
is preferably 4.0 or more, and more preferably, the washing is repeated and flushed
out until the pH becomes 5.5 or more, further more preferably 6.5 or more.
[0020] In the above step 3), an aqueous lithium hydroxide solution, an aqueous sodium hydroxide
solution, an aqueous magnesium hydroxide solution, an aqueous potassium hydroxide
solution, an aqueous calcium hydroxide solution, an aqueous barium hydroxide solution
etc., can be used as the "aqueous alkaline solution", the aqueous sodium hydroxide
solution and the aqueous potassium hydroxide solution are preferable from the viewpoint
of ready availability and removal capability of the polymers. The concentration of
the aqueous alkaline solution is not particularly limited, but preferably 0.1 to 30%
by mass, more preferably 1.0 to 20% by mass, and further more preferably 2.0 to 15
% by mass. Usually, said alkaline solution is circulated in the housing and promotes
removal of the polymer adhered to the filter.
[0021] In the above step 4), the pH of water after flushing out the residual alkali is preferably
8.0 or less, and more preferably, the washing is repeated and flushed out until the
pH becomes 7.5 or less.
[0022] In the above step 5), the filter may be brought in contact with the acrylic acid
for regeneration, and the contact method is not particularly limited. For example,
the acrylic acid for regeneration is poured from the upper part of the filter, or
the filter is immersed into the acrylic acid for regeneration, etc. When the filter
is immersed in the acrylic acid for regeneration, it is only permitted to immerse
in stationary acrylic acid for regeneration or in some case, to immerse in circulated
acrylic acid for regeneration. Furthermore, at a given point in contacting with the
acrylic acid for regeneration, mechanical method such as rubbing off may be added.
It is desirable that the above acrylic acid for regeneration has a purity which can
wash efficiently the filter, and the APHA of the acrylic acid for regeneration is
preferably 10 or less, more preferably 5 or less, and further more preferably 2 or
less. Furthermore, the metal content of the acrylic acid for regeneration is preferably
3 ppm by mass or less, more preferably 1 ppm by mass or less, and further more preferably
0.1 ppm by mass or less. In addition, the temperature of the acrylic acid for regeneration
is in the range of 14 to 80°C in consideration of the melting point of acrylic acid
and the heat resistance-temperature of the filter etc., and more preferably 15 to
40°C in consideration of the polymerizability of the acrylic acid.
[0023] The time during which a filter is brought in contact with the acrylic acid for regeneration
under the above condition, is preferably at least 1 hour or more, and more preferably
2 hours or more. When the filter is immersed in acrylic acid for regeneration for
1 hour or more, metal oxides on the filter can be fully removed and the amount of
the above-mentioned off-specification product decreases.
[0024] By using the filter after performed the above steps 1 to 5 for the filtering step,
deterioration of the color tone of the purified acrylic acid can be prevented efficiently.
[0025] The above regeneration of the filter may be performed out of the housing by removing
the filter. In addition, filter after regeneration can be used for a filtering step
as is, without performing further operation, and can continue the production of acrylic
acid efficiently.
EXAMPLE
[0026] Hereinafter, the present invention is explained still more specifically using examples
and comparative examples. However, the technical range of the present invention is
not necessarily restricted only to the following examples.
[0027] The color tone and the metal content (since the metal is mainly iron, the iron content
is described) were measured by the following methods.
Color tone: Absorption colorimeter ("ColorPlus" of Color grade meter manufactured
by SIGRIST-PHOTOMETER AG) was installed in the liquid sending line, and Hazen color
number (APHA) was measured online (wavelength at measurement, 365 nm; wavelength at
comparison, 650 nm).
Iron contents: After diluting the acrylic acid with ultrapure water by 5 times, the
sample was analyzed with an ICP (Inductively Coupled Plasma) Atomic Emission Spectrometer.
[0028] The purified acrylic acid used here was prepared as follows.
<Step of absorbing acrylic acid>
[0029] The reactive gas obtained by a catalytic gas phase oxidation reaction of propylene
by the same method as in the case of Example 1 of
JP-A-2005-15478 was contacted with the aqueous solution for absorbing, and from the bottom of the
absorption column, an acrylic acid solution with a composition of acrylic acid 90.0%
by mass, water 3.2% by mass, acetic acid 1.9% by mass, maleic acid 0.6% by mass, acrylic
acid dimer 1.5% by mass, aldehydes 0.4% by mass, hydroquinone 0.1% by mass, and other
impurities 2.3% by mass was obtained. It should be noted that the temperature at the
bottom of absorption column at this time, i.e., the temperature of the acrylic acid
solution removed from the absorption column, was 91°C.
<Step of crystallization of acrylic acid>
[0030] After cooling the above acrylic acid solution, it was supplied to a crystallizer,
and dynamic crystallization was repeated 4 times to purify. Dynamic crystallization
was performed using the crystallizer according to the crystallizer described in
JP-B-53-41637. That is, an apparatus which is equipped with a reservoir in the lower portion and
enables a liquid in the reservoir to transfer in a metal tube having a length of 6
m and an inner diameter of 70 mm by use of a circulation pump to the upper part of
the tube and to flow along the inner wall surface of the tube as falling film is used.
The surface of the tube is composed of a double jacket and the jacket is controlled
so as to be a constant temperature by use of a thermostat. A cycle of dynamic crystallization
was performed according to the following procedures.
1) Crystallization: the crude acrylic acid was supplied to the reservoir, flowed along
of the inner wall surface of the tube as falling film by a circulation pump, and the
jacket temperature was reduced below the freezing point, to crystallize about 60 to
90% by mass of acrylic acid on the wall surface.
2) Sweating: the circulation pump was stopped, and the jacket temperature was raised
to near the freezing point, and about 2 to 20% of the acrylic acid crystal was sweated.
After sweating, residual melted liquid was pumped out with the pump.
3) Melting: the jacket temperature was raised to the freezing point or more, and the
crystal was melted and then pumped out with the pump.
[0031] In the above operations, the temperature and freezing point depend on each process
carried out.
[0032] By the above, the purified acrylic acid having a purity of 99.93% by mass was obtained.
Composition of other elements in said purified acrylic acid was water 100 ppm by mass,
acetic acid 475 ppm by mass, maleic acid 2 ppm by mass, acrylic acid dimer 30 ppm
by mass, and the aldehydes 0.4 ppm by mass. It should be noted that methoquinone was
added to said purified acrylic acid as a stabilizer so as to be 200 ppm by mass.
[0033] The APHA value of the above purified acrylic acid was 2, and the iron content was
0.1 ppm by mass.
[0034] The above purified acrylic acid was passed into the cartridge housing which stored
the cartridge filter (TCP-1 manufactured by ADVANTEC, a pleats cartridge filter made
of PP, pore size with 1 µm), and it was supplied to the storage tank. The linear velocity
of the purified acrylic acid in the filter part at this time was 0.01 m/s.
[0035] When the differential pressure in the filter part became 0.1 MPa, the supply of the
purified acrylic acid was stopped, and the filter was regenerated with the following
procedures.
<Example 1>
[0036]
- 1) The acrylic acid which remained in the housing was removed. Said acrylic acid was
recovered and was circulated to the absorbing step for acrylic acid.
- 2) Water was supplied to the housing and water washing of the filter was performed.
That is, after filling the interior of the housing with water, continual operation
of removing water immediately was repeated until the pH of the water after washing
became 6.5. Part of the washing water in the early stage of washing was circulated
to the above-mentioned absorbing step for acrylic acid after recovery, and the subsequent
residual washing water was removed from the system.
- 3) Aqueous sodium hydroxide solution 10 mass % was supplied to the housing, and alkaline
washing of the filter was performed. The alkaline washing was performed as follows.
That is, after filling the housing with aqueous sodium hydroxide solution and circulating
this aqueous sodium hydroxide solution inside the housing for a while with a pump,
the aqueous alkaline solution was removed from housing. Such operations were repeated
3 times. The aqueous alkaline solution after washing was removed from the system.
- 4) Water was supplied in the housing and water washing of a filter was performed.
That is, after filling the housing with water, continuous operation of removing the
water immediately was repeated until the pH of the liquid after washing became 7.5.
- 5) Purified acrylic acidwas supplied in the housing as acrylic acid for regeneration,
and the housing was filled with said acrylic acid. After the filter was immersed for
1 hour, holding the temperature of the above-mentioned acrylic acid for regeneration
at 40°C, said acrylic acid was removed. The removed acrylic acid was recovered and
circulated to the above-mentioned crystallization process for acrylic acid.
[0037] After regenerating the filter by the above-mentioned procedure, the supply of the
purified acrylic acid to this filter was restarted. The color tone of the acrylic
acid which flowed out of the filter immediately after the restart was 5 (APHA), and
the iron content was 1.3 ppm by mass. Two hours afterward, the color tone became 2
(APHA), the iron content became 0.1 ppm by mass, and returned to the initial value
of the purified acrylic acid. Then, the supply of the filtered acrylic acid to the
storage tank was started. Acrylic acid between the re-starting of filtration and the
time when the color tone and the iron content returned to the initial levels of the
purified acrylic acid was recovered as an off-spec product, and it was then circulated
to the above-mentioned crystallization process.
Example 2
[0038] A procedure similar to that in Example 1 was carried out except that the temperature
of the acrylic acid for regeneration was 25°C in the above-mentioned step 5).
Example 3
[0039] A procedure similar to that in Example 1 was carried out except that the temperature
of the acrylic acid for regeneration was 15°C in the above-mentioned step 5).
Example 4
[0040] A procedure similar to that in Example 2 was carried out except that the immersion
time in the acrylic acid for regeneration was 2 hrs in the above-mentioned step 5).
Example 5
[0041] A procedure similar to that in Example 2 was carried out except that the immersion
time in the acrylic acid for regeneration was 3 hrs in the above-mentioned step 5).
Example 6
[0042] A procedure similar to that in Example 2 was carried out except that the immersion
time in the acrylic acid for regeneration was 6 hrs in the above-mentioned step 5).
Example 7
[0043] A procedure similar to that in Example 2 was carried out except that the immersion
time in the acrylic acid for regeneration was 9 hrs in the above-mentioned step 5).
Example 8
[0044] A procedure similar to that in Example 2 was carried out except that the immersion
time in the acrylic acid for regeneration was 12 hrs in the above-mentioned step 5).
Comparative example 1
[0045] A procedure similar to that in Example 2 was carried out except that the acrylic
acid for regeneration was not filled in the above-mentioned step 5).
Comparative example 2
[0046] A procedure similar to that in Example 2 was carried out except that the immersion
time in the acrylic acid for regeneration was 0.5 hrs in the above-mentioned step
5).
Comparative example 3
[0047] A procedure similar to that in Example 1 was carried out except that the immersion
time in the acrylic acid for regeneration was 0.5 hrs in the above-mentioned step
5).
Comparative example 4
[0048] A procedure similar to that in Example 3 was carried out except that the immersion
time in the acrylic acid for regeneration was 0.5 hrs in the above-mentioned step
5).
[0049] The above result is shown in Table 1. It should be noted that the "Required time"
in Table 1 means the following total time; after restarting filtration with a regenerated
filter, until the color tone and the metal content of the filtered acrylic acid reaches
the initial color tone (APHA 2) and metal content (mainly, the iron content) (0.1
ppm by mass) of the purified acrylic acid, and then it becomes possible to transfer
this product to storage tanks.
[0050]
Table 1
|
Immersion time (hr) |
Temperature of acrylic acid for regeneration (°C) |
Quality immediately after restart |
Required time (hr) |
APHA |
Iron content (ppm) |
Example 1 |
1 |
40 |
5 |
1.3 |
2.0 |
Example 2 |
1 |
25 |
8 |
2.4 |
3.7 |
Example 3 |
1 |
15 |
9 |
3.0 |
4.6 |
Example 4 |
2 |
25 |
6 |
1.5 |
2.4 |
Example 5 |
3 |
25 |
5 |
1.2 |
1.8 |
Example 6 |
6 |
25 |
4 |
0.6 |
1.0 |
Example 7 |
9 |
25 |
3 |
0.3 |
0.5 |
Example 8 |
12 |
25 |
2 |
0.2 |
0.25 |
Comparative example 1 |
- |
25 |
22 |
8.0 |
12.5 |
Comparative example 2 |
0.5 |
25 |
19 |
6.8 |
10.6 |
Comparative example 3 |
0.5 |
40 |
18 |
6.6 |
10.3 |
Comparative example 4 |
0.5 |
15 |
19 |
6.9 |
10.8 |
[0051] From Table 1, it is understood that when the temperature of the acrylic acid for
regeneration employed for immersion is higher, and/or the immersion time in this acrylic
acid is longer, the quality immediately after the restart is preferable, and the required
time is shortened.